The 2004 Indian Ocean tsunami was caused by a major earthquake under the seafloor north of Aceh in Sumatra. Scientists say another massive undersea earthquake is long overdue beneath the Mentawai islands in Indonesia and could trigger another deadly tsunami anytime.

HOW TSUNAMIS OCCUR
In the Sumatra area, the tectonic plates meet in a subduction zone, where the boundaries of one plate are forced beneath the other plate. The Indo-Australian plate is sliding northeastward (at about 7 cm per year) and dipping under the Eurasian plate, along a fault line called the Sunda megathrust which runs southwest from Myanmar down Indonesia towards Timor.

Tremendous geological strain builds up, over many decades or even longer than a century, until a section of the megathrust gives way. This rupture causes the oceanic plates beneath Sumatra to lurch forward suddenly, by many metres, in a big earthquake.

If the ocean floor ruptures, it suddenly moves a massive amount of water. This is what happened in the earthquake that caused the deadly Indian Ocean tsunamis of December 2004. The water displaced by the Aceh earthquake was like tipping out the volume of Sydney Harbour within a few minutes. Major quakes that rupture the ocean floor are usually shallow quakes occuring at a depth of less than 70 km (44 miles). The quake that caused the 2004 tsunami was 30 km below the seafloor.

TSUNAMIS RISE UP
On the ocean surface, tsunamis start as an insignificant ripple capable of passing under a ship unnoticed, but they become giants as they approach land and the ocean becomes shallow. A tsunami is not a single wave, but a series of waves. They can travel across the ocean at speeds of up to 1,000 km (620 miles) an hour, the speed of a jet aircraft. The vast size of the Pacific Ocean and the large earthquakes associated with the "Ring of Fire" combine to produce deadly tsunamis in the Asia-Pacfic. A tsunami can travel across the Pacific Ocean in less than a day.

As the waves approaches land, the ocean recedes dramatically exposing reefs, as the waves draw the water out. As the trough of the wave drags along the sea floor, slowing it down, the crest rises up dramatically and sends a giant wall of whitewater onto land. The first wave may not be the biggest. The destructive force of a tsunami comes not from the height of the wave, but from the volume of water moving. It is as if the ocean floods the coast, smashing everything in its path, and then just as quickly recedes. Many people who survive the initial wave impact are washed out to sea as the tsunami recedes.